Also take a look at L.B.Jackson's text book "Digital filters and Signal
processing" Kluwer publishing, Ch.4. It has a unity gain resonator which
can be tuned by varying just one coefficient. Incidentally that was the
first digital filter ever built (with J.F.kaiser, and J.McDonald) some
time in 1964?? RK

RK, I just stumbled on some info about that digital filter you
referred to, in the DSP History column of the Nov. 2004 issue of
Signal Processing Magazine (of the IEEE Signal Processing society),
written by Leland Jackson himself.

Jackson didn't get to Bell Labs until late 1966. His first project,
a touch-tone decoder with digital filters using multiplexed
bit-serial implementation, was working by 1967, I think, and the
publication was out in 1968.

Jackson says that as far as he knows it was "the first realization of
a fully programmable digital filter in hardware form." One could say
the "fully programmable" is an overstatement, since it was fixed
function with coefficients in ROM, but the point is that the filter
stages themselves had programmable coefficients. Earlier digital
filters that he was aware of, "fixed clutter rejection digital
filters in moving-target-indicator radars," were not programmable in
that sense, with hardwired coefficients.

It's an interesting column. I recommend it to anyone interested in
DSP history. Maybe that's where you got the info you recalled?